This invention relates generally to the field of digitally controlled printing devices, and in particular to continuous inkjet printers in which a liquid ink stream breaks into droplets, some of which are selectively deflected. Either the deflected droplets or the non-deflected droplets can be printed on a print medium with the droplets having corrected print locations.
Traditionally, digitally controlled color printing capability is accomplished by one of two technologies. Both require independent ink supplies for each of the colors of ink provided. Ink is fed through channels formed in the printhead. Each channel includes a nozzle from which droplets of ink are selectively extruded and deposited.upon a medium. Typically, each technology requires separate ink delivery systems for each ink color used in printing. Ordinarily, the three primary subtractive colors, i.e. cyan, yellow and magenta, are used because these colors can produce, in general, up to several million perceived color combinations. In the construction of printers incorporating either technology, the printhead, typically, includes a plurality of nozzles arranged in a linear array. The printhead is typically scanned in a fast scan direction, substantially perpendicular to the row of nozzles, over a print medium. Additionally, the printhead may be stepped in a slow scan direction, substantially perpendicular to the fast scan direction, before the fast scan is repeated.
The first technology, commonly referred to as xe2x80x9cdrop-on-demandxe2x80x9d ink jet printing, provides ink droplets for impact upon a recording surface using a pressurization actuator (thermal, piezoelectric, etc.). Selective activation of the actuator causes the formation and ejection of a flying ink droplet that crosses the space between the printhead and the print media and strikes the print media. The formation of printed images is achieved by controlling the individual formation of ink droplets, as is required to create the desired image. Typically, a slight negative pressure within each channel keeps the ink from inadvertently escaping through the nozzle, and also forms a slightly concave meniscus at the nozzle, thus helping to keep the nozzle clean.
Conventional xe2x80x9cdrop-on-demandxe2x80x9d ink jet printers utilize a pressurization actuator to produce the ink jet droplet at orifices of a print head. Typically, one of two types of actuators are used including heat actuators and piezoelectric actuators. With heat actuators, a heater, placed at a convenient location, heats the ink causing a quantity of ink to phase change into a gaseous steam bubble that raises the internal ink pressure sufficiently for an ink droplet to be expelled. With piezoelectric actuators, an electric field is applied to a piezoelectric material possessing properties that create a mechanical stress in the material causing an ink droplet to be expelled. The most commonly produced piezoelectric materials are ceramics, such as lead zirconate titanate, barium titanate, lead titanate, and lead metaniobate. While heat actuators and piezoelectric actuators have been long used in drop-on-demand printing, they suffer from a lack of precise control of the placement of drops on the print medium, which is a critical parameter for image quality. With heat actuators, the nozzles or heaters may become contaminated due to thermally induced decomposition of ink, thereby causing drop misplacement errors. With piezoelectric actuators, the properties of the piezoelectric material may change with use and/or failure of the ink fluid meniscus to reproducibly engage the nozzle during each drop firing cause drop misplacement errors. In either case, highly visible artifacts may be produced particularly when misplacement errors occur repeatedly on the print medium. The artifacts are most apparent when the placement errors are perpendicular to the fast scan direction because the errors are repeated in a line. This type of image artifact is well known in the inkjet printer art.
U.S. Pat. No. 4,914,522 issued to Duffield et al., on Apr. 3, 1990 discloses a drop-on-demand ink jet printer that utilizes air pressure to produce a desired color density in a printed image. Ink in a reservoir travels through a conduit and forms a meniscus at an end of an inkjet nozzle. An air nozzle, positioned so that a stream of air flows across the meniscus at the end of the ink nozzle, causes the ink to be extracted from the nozzle and atomized into a fine spray. The stream of air is applied at a constant pressure through a conduit to a control valve. The valve is opened and closed by the action of a piezoelectric actuator. When a voltage is applied to the valve, the valve opens to permit air to flow through the air nozzle. When the voltage is removed, the valve closes and no air flows through the air nozzle. As such, the ink dot size on the image remains constant while the desired color density of the ink dot is varied depending on the pulse width of the air stream.
The second technology, commonly referred to as xe2x80x9ccontinuous streamxe2x80x9d or xe2x80x9ccontinuousxe2x80x9d inkjet printing, uses a pressurized ink source which produces a continuous stream of ink droplets. Conventional continuous ink jet printers utilize electrostatic charging devices that are placed close to the point where a filament of working fluid breaks into individual ink droplets. The ink droplets are electrically charged and then directed to an appropriate location by deflection electrodes having a large potential difference. When no print is desired, the ink droplets are deflected into an ink capturing mechanism (catcher, interceptor, gutter, etc.) and either recycled or disposed of. When print is desired, the ink droplets are not deflected and allowed to strike a print media. Alternatively, deflected ink droplets may be allowed to strike the print media, while non-deflected ink droplets are collected in the ink capturing mechanism.
Typically, continuous inkjet printing devices are faster than droplet on demand devices. However, each color printed requires an individual droplet formation, deflection, and capturing system.
Conventional continuous ink jet printers utilize electrostatic charging devices and deflector plates, they require many components and large spatial volumes in which to operate. This results in continuous ink jet printheads and printers that are complicated, have high energy requirements, are difficult to manufacture, and are difficult to control. As charged drops repel one another, drop placement accuracy suffers, particularly in a line parallel to the linear array of nozzles. The artifacts are most apparent when the placement errors are perpendicular to the fast scan direction because the errors are repeated in a line over a substantial distance on the recording medium. Examples of conventional continuous ink jet printers include U.S. Pat. No. 1,941,001, issued to Hansell, on Dec. 26, 1933; U.S. Pat. No. 3,373,437 issued to Sweet et al., on Mar. 12, 1968; U.S. Pat. No. 3,416,153, issued to Hertz et al., on Oct. 6; 1963; U.S. Pat. No. 3,878,519, issued to Eaton, on Apr. 15, 1975; and U.S. Pat. No. 4,346,387, issued to Hertz, on Aug. 24, 1982.
U.S. Pat. No. 3,709,432, issued to Robertson, on Jan. 9, 1973, discloses a method and apparatus for stimulating a filament of working fluid causing the working fluid to break up into uniformly spaced ink droplets through the use of transducers. The lengths of the filaments before they break up into ink droplets are regulated by controlling the stimulation energy supplied to the transducers, with high amplitude stimulation resulting in short filaments and low amplitudes resulting in long filaments. A flow of air is generated across the paths of the fluid at a point intermediate to the ends of the long and short filaments. The air flow affects the trajectories of the filaments before they break up into droplets more than it affects the trajectories of the ink droplets themselves. By controlling the lengths of the filaments, the trajectories of the ink droplets can be controlled, or switched from one path to another. As such, some ink droplets may be directed into a catcher while allowing other ink droplets to be applied to a receiving member.
While this method does not rely on electrostatic means to affect the trajectory of droplets it does rely on the precise control of the break off points of the filaments and the placement of the air flow intermediate to these break off points. Such a system is difficult to control and to manufacture. Furthermore, the physical separation or amount of discrimination between the two droplet paths is small further adding to the difficulty of control and manufacture. As such, these printheads suffer from a lack of precise control of the placement of drops on the print medium which can produce visible image artifacts. Again, the artifacts are most apparent when the placement errors are perpendicular to the fast scan direction.
U.S. Pat. No. 4,190,844, issued to Taylor, on Feb. 26, 1980, discloses a continuous ink jet printer having a first pneumatic deflector for deflecting non-printed ink droplets to a catcher and a second pneumatic deflector for oscillating printed ink droplets. A printhead supplies a filament of working fluid that breaks into individual ink droplets. The ink droplets are then selectively deflected by a first pneumatic deflector, a second pneumatic deflector, or both. The first pneumatic deflector is an xe2x80x9con/offxe2x80x9d or an xe2x80x9copen/closedxe2x80x9d type having a diaphram that either opens or closes a nozzle depending on one of two distinct electrical signals received from a central control unit. This determines whether the ink droplet is to be printed or non-printed. The second pneumatic deflector is a continuous type having a diaphram that varies the amount a nozzle is open depending on a varying electrical signal received the central control unit. This oscillates printed ink droplets so that characters may be printed one character at a time. If only the first pneumatic deflector is used, characters are created one line at a time, being built up by repeated traverses of the printhead.
While this method does not rely on electrostatic means to affect the trajectory of droplets it does rely on the precise control and timing of the first (xe2x80x9copen/closedxe2x80x9d) pneumatic deflector to create printed and non-printed ink droplets. Such a system is difficult to manufacture and accurately control resulting in at least the ink droplet build up discussed above. Furthermore, the physical separation or amount of discrimination between the two droplet paths is erratic due to the precise timing requirements increasing the difficulty of controlling printed and non-printed ink droplets resulting in poor ink droplet trajectory control. The erratic trajectories cause random errors in drop placement on the print medium which reduces image quality, while manufacturing defects cause systematic errors in drop placement. Both errors produce highly visible artifacts, particularly when the artifacts occur repeatedly over large distances on the print medium.
Additionally, using two pneumatic deflectors complicates construction of the printhead and requires more components. The additional components and complicated structure require large spatial volumes between the printhead and the media, increasing the ink droplet trajectory distance. Increasing the distance of the droplet trajectory decreases droplet placement accuracy and affects the print image quality. Again, there is a need to minimize the distance the droplet must travel before striking the print media in order to insure high quality images. Pneumatic operation requiring the air flows to be turned on and off is necessarily slow in that an inordinate amount of time is needed to perform the mechanical actuation as well as settling any transients in the air flow.
U.S. Pat. No. 6,079,821, issued to Chwalek et al., on Jun. 27, 2000, discloses a continuous inkjet printer that uses actuation of asymmetric heaters to create individual ink droplets from a filament of working fluid and deflect thoses ink droplets. A printhead includes a pressurized ink source and an asymmetric heater operable to form printed ink droplets and non-printed ink droplets. Printed ink droplets flow along a printed ink droplet path ultimately striking a print media, while non-printed ink droplets flow along a non-printed ink droplet path ultimately striking a catcher surface. Non-printed ink droplets are recycled or disposed of through an ink removal channel formed in the catcher.
While the ink jet printer disclosed in Chwalek et al. works extremely well for its intended purpose, using a heater to create and deflect ink droplets increases the energy and power requirements of this device. This can cause ink to be thermally decomposed on the heaters resulting in ink contamination on or around the heater and/or nozzle. Ink contamination can reduce drop placement accuracy by interfering with the ink meniscus profile of the ejected ink stream and by altering the thermal efficiency of the heaters.
In both drop-on-demand and continuous ink jet printers, the visibility of artifacts caused by drop placement errors can be reduced by using only a portion of available nozzles chosen at random during each fast scan, as is practiced in currently available conventional ink jet printers. However, using random nozzles requires making many scans of the printhead over the same, or nearly the same, location on the print medium and thus reduces printer productivity.
For nozzles which are not defective, the paths of drops ejected from a row of equally spaced nozzles should be parallel. A defective nozzle eject drops whose path is not parallel to the paths of drops ejected from nozzles which are not defective. It is possible to determine whether nozzles are defective by viewing ejected drops from a direction perpendicular to the drop paths, that is by viewing from the fast scan direction. While it is possible to use conventional printheads and print using only the defective nozzles, this reduces printhead yield and greatly increases printhead cost. The problem is particularly acute for printheads having a very large number of nozzles.
It can be seen that there is a need to provide an inkjet printhead and printer of simple construction having reduced energy and power requirements and improved placement accuracy of individual ink drops on the recording medium. In particular, there is a need to provide an inkjet printhead and printer of simple construction with nozzles which can be operated in a way to avoid systematic drop misplacement errors by providing precise control of the placement of ink drops in the slow scan direction. Alternatively stated, there is a need to inexpensively provide a printhead having simplified control of individual ink droplets which ejects drops having paths of travel that are parallel when viewed from the fast scan printing direction.
It is an object of the present invention is to simplify construction of a continuous ink jet printhead and printer having improved placement accuracy of individual ink drops in order to render images of high quality.
Another object of the present invention is to reduce energy and power requirements of a continuous inkjet printhead and printer having improved placement accuracy of individual ink drops in the slow scan direction.
Yet another object of the present invention is to provide a continuous ink jet printhead and printer capable of rendering high resolution images with reduced image artifacts using large volumes of ink.
Yet another object of the present invention is to improve the reliability of a continuous ink jet printhead.
Yet another object of the present invention is to simplify construction and operation of a continuous ink jet printer suitable for printing high quality images having reduced artifacts due to systematic errors of drop placement.
Yet another object of the present invention is to provide a continuous ink jet printhead and printer capable of printing images having reduced image artifacts with a wide variety of inks on a wide variety of materials.
According to a feature of the present invention, an apparatus for printing an image includes a source of ink. A droplet forming mechanism is operable in a first state to form droplets from the source having a first volume traveling along a first desired path and in a second state to form droplets from the source having a second volume traveling along the first desired path. The droplet forming mechanism is positioned proximate the source. A first system selectively applies a first force to the source such that selected droplets formed from the source by the droplet forming mechanism travel along a second desired path. The first system is positioned proximate the source. A second system applies a second force to the droplets traveling along at least one of the first desired path and the second desired path. The second force is applied in a direction such that the droplets having the first volume diverge from the droplets having the second volume.
According to another feature of the present invention, a printhead includes a droplet forming mechanism is operable in a first state to form droplets having a first volume traveling along a first desired path and in a second state to form droplets having a second volume traveling along the first desired path. A droplet steering system is positioned relative to the droplet forming mechanism which selectively applies a first force to the droplets formed from the source such that selected droplets formed from the source travel along a second desired path. A droplet deflector system is positioned relative to the droplet forming mechanism which applies a second force to the droplets traveling along at least one of the first desired path and the second desired path. The second force is applied in a direction such that the droplets having the first volume diverge from at least one of the first desired path and the second desired path.
According to another feature of the present invention, an inkjet printer includes a source of ink. A printhead having a droplet forming mechanism is operable in a first state to form droplets from the source having a first volume traveling along a desired path and in a second state to form droplets from the source having a second volume traveling along the desired path. A droplet steering system is positioned relative to the droplet forming mechanism which selectively applies a first force to the droplets formed from the source such that the droplets formed from the source travel along a second desired path. A droplet deflector system is positioned relative to the droplet forming mechanism which applies a second force to the droplets traveling along at least one of the first desired path and the second desired path. The second force is applied in a direction such that the droplets having the first volume diverge from at least one of the first desired path and the second desired path.
According to another feature of the present invention, a method of printing an image having corrected ink droplet placement includes forming droplets having a first volume traveling along a first desired path; forming droplets having a second volume traveling along the first desired path; causing the droplets having the first volume to diverge from the first desired path; collecting the droplets having one of the first volume and the second volume; allowing the droplets having the other of the first volume and the second volume to impinge upon a recording media; determining when the droplets having one of the first volume and the second volume begin traveling along an undesired path; and correcting the droplets having one of the first volume and the second volume such that the droplets resume traveling along the desired path.
According to another feature of the present invention, a method of correcting droplet placement error in a plurality of nozzles aligned in a row includes forming droplets from a first nozzle traveling along a first desired path; forming droplets from a second nozzle traveling along a second desired path, the second desired path being substantially parallel to the first desired path as viewed in a direction perpendicular to the row and perpendicular to a fast scan direction; determining when the second desired path is other than parallel to the first desired path; and causing the second desired path to resume being parallel to the first desired path.